STAB(5) MachTen Programmer’s Manual STAB(5)

NAME
stab - symbol table types

SYNOPSIS
#include <stab.h>

DESCRIPTION
The file <stab.h> defines some of the symbol table n_type field values
for a.out files. These are the types for permanent symbols (i.e. not lo-
cal labels, etc.) used by the old debugger sdb and the Berkeley Pascal
compiler pc(1). Symbol table entries can be produced by the .stabs as-
sembler directive. This allows one to specify a double-quote delimited
name, a symbol type, one char and one short of information about the sym-
bol, and an unsigned long (usually an address). To avoid having to pro-
duce an explicit label for the address field, the .stabd directive can be
used to implicitly address the current location. If no name is needed,
symbol table entries can be generated using the .stabn directive. The
loader promises to preserve the order of symbol table entries produced by
.stab directives. As described in a.out(5), an element of the symbol
table consists of the following structure:

/*
* Format of a symbol table entry.
*/

struct nlist {
union {
char *n_name; /* for use when in-core */
long n_strx; /* index into file string table */
} n_un;
unsigned char n_type; /* type flag */
char n_other; /* unused */
short n_desc; /* see struct desc, below */
unsigned n_value; /* address or offset or line */
};

The low bits of the n_type field are used to place a symbol into at most
one segment, according to the following masks, defined in <a.out.h>. A
symbol can be in none of these segments by having none of these segment
bits set.

/*
* Simple values for n_type.
*/

#define N_UNDF 0x0 /* undefined */
#define N_ABS 0x2 /* absolute */
#define N_TEXT 0x4 /* text */
#define N_DATA 0x6 /* data */
#define N_BSS 0x8 /* bss */

#define N_EXT 01 /* external bit, or’ed in */

The n_value field of a symbol is relocated by the linker, ld(1) as an ad-
dress within the appropriate segment. N_value fields of symbols not in
any segment are unchanged by the linker. In addition, the linker will
discard certain symbols, according to rules of its own, unless the n_type
field has one of the following bits set:

/*
* Other permanent symbol table entries have some of the N_STAB bits set.
* These are given in <stab.h>
*/

#define N_STAB 0xe0 /* if any of these bits set, don’t discard */

This allows up to 112 (7 * 16) symbol types, split between the various
segments. Some of these have already been claimed. The old symbolic de-
bugger, sdb, uses the following n_type values:

#define N_GSYM 0x20 /* global symbol: name,,0,type,0 */
#define N_FNAME 0x22 /* procedure name (f77 kludge): name,,0 */
#define N_FUN 0x24 /* procedure: name,,0,linenumber,address */
#define N_STSYM 0x26 /* static symbol: name,,0,type,address */
#define N_LCSYM 0x28 /* .lcomm symbol: name,,0,type,address */
#define N_RSYM 0x40 /* register sym: name,,0,type,register */
#define N_SLINE 0x44 /* src line: 0,,0,linenumber,address */
#define N_SSYM 0x60 /* structure elt: name,,0,type,struct_offset */
#define N_SO 0x64 /* source file name: name,,0,0,address */
#define N_LSYM 0x80 /* local sym: name,,0,type,offset */
#define N_SOL 0x84 /* #included file name: name,,0,0,address */
#define N_PSYM 0xa0 /* parameter: name,,0,type,offset */
#define N_ENTRY 0xa4 /* alternate entry: name,linenumber,address */
#define N_LBRAC 0xc0 /* left bracket: 0,,0,nesting level,address */
#define N_RBRAC 0xe0 /* right bracket: 0,,0,nesting level,address */
#define N_BCOMM 0xe2 /* begin common: name,, */
#define N_ECOMM 0xe4 /* end common: name,, */
#define N_ECOML 0xe8 /* end common (local name): ,,address */
#define N_LENG 0xfe /* second stab entry with length information */

where the comments give sdb conventional use for .stab s and the n_name,
n_other, n_desc, and n_value fields of the given n_type. Sdb uses the
n_desc field to hold a type specifier in the form used by the Portable C
Compiler, cc(1); see the header file pcc.h for details on the format of
these type values.

The Berkeley Pascal compiler, pc(1), uses the following n_type value:

#define N_PC 0x30 /* global pascal symbol: name,,0,subtype,line */

and uses the following subtypes to do type checking across separately
compiled files:

1 source file name
2 included file name
3 global label
4 global constant
5 global type
6 global variable
7 global function
8 global procedure
9 external function
10 external procedure
11 library variable
12 library routine